Answer
$8000c\cdot sin^2\theta cos\theta$ where $c$ is a constant.
Work Step by Step
Step 1. Use the figure given in Exercise 67, let $w$=width and $d$=depth, $S$=strength of the beam.
Step 2. As the strength of a beam is proportional to the width and the square of the depth, we have
$S=cwd^2$ where $c$ is a constant.
Step 3. Recall the results from Exercise 67, $w=20cos\theta$ and $d=20sin\theta$, we get
$S=cwd^2=20c\cdot cos\theta\times(20sin\theta)^2=8000c\cdot sin^2\theta cos\theta$
